Due to the increasing number of functions integrated into single semiconductor devices, such as the Micro Processor Unit (MPU) used in computers, heat dissipation becomes an important consideration and necessity. Heat sink structures and cooling fans are mounted directly on top of the MPU to dissipate heat. This direct mounting of heat dissipation devices is in addition to cooling fans associated with the power supply in the computer.
In present day processors, the MPUs execute all the functions of a 10 year old CPU (Central Processing Unit) of a mainframe computer. These MPUs execute instructions, fetch, decode, execute floating point and integer arithmetic as well as memory management.
Also a consideration, particularly in new high speed Central Processing Units, is the electrical performance of the circuitry at high. Devices with short leads, such as Surface Mount, TAB, and Pin Grid Array have been used. However, there are still requirements for electrical conductors on the circuit board to connect to other components, including cache memory devices. It is desirable to have connections between the MPU and cache memory devices to be as short as possible since access to these memories hinders the throughput of the MPU.
In order to arrange the largest number of components onto a Printed Wiring Board (PWB), a new packaging design which utilizes the third dimension is implemented.